The invention concerns a process for surface measurement using confocal microscopy in a reflection process, especially for measuring the surface profile of teeth in the untreated and in the treated or drilled conditions.
This is essentially a process for measurement of surfaces of any type and any contour. Various processes for surface measurement are already known in practice.
For instance, a line of light can be projected on the object with a light sectioning sensor, and observed at an angle with a CCD camera. The geometric deformation of the line is measured. The height differences on the object are computed from this deformation. By moving the object under the sensor--perpendicularly to the light line--and by repeated measurement, a surface form can be measured or determined from a series of profiles.
The light-sectioning sensor is indeed a simply designed and thus a robust sensor, but the oblique lighting which it requires causes unilateral shading of steep surfaces. That causes asymmetries in the imaging, or inaccuracies. Furthermore, error can be introduced into the measurements because of scattering of light from various depths of, for instance, an at least partially transparent tooth material.
Furthermore, it is already known in practice that surfaces can be scanned with confocal microscopy so as to generate three-dimensional pictures of the surface. In this respect, it is only necessary to refer to an article by J. Engelhardt and W. Knebel in Physik in unserer Zeit' [Physics in Our Time], Vol. 24, 1993, No. 3, titled "Confokale Laserscanning-Mikroskopie" [Confocal Laser Scanning Microscopy], and one by D. K Hamilton and T. Wilson in Appl. Phys., B27, 211-213, 1982, titled "Three-dimensional Surface Measurement Using the Confocal Scanning Microscope". Confocal microscopy is very specially suited to surface measurements of tooth surfaces, because this process images only those structures which are exactly in the focal plane of the microscope objective. Thus measurement errors due to the partially transparent tooth material are effectively avoided. To be sure, the method of reflection measurement with the usual confocal microscope fails at steep transitions or flanks if their angle is greater than the aperture angle of the objective, because then the reflection no longer enters the objective, and is lost for evaluation. (See P. C. Cheng and R. G. Summers in: "Handbook of Biological Confocal Microscopy", Plenum Press, New York, 1989, Chapter 17.)